Numerous investigators have suggested that during both Ca ions regulated thin filament activity and actin-myosin interaction, structural or conformational changes occur in the actin polymer. The objective of this research project is to investigate the occurrence of conformational changes in actin: (a) during its G to F transformation and (b) while it is in the polymer state, and to ascertain which of the conformational states will interact with myosin. Certain agents which are known to affect the stability of the polymer or alter the monomer-polymer equilibrium will be employed in the investigation. The susceptibilities of the various conformational states to proteolytic digestion will be correlated with viscosity measurements and the ability of the state to activate the myosin ATPase. New information will be forthcoming about the existence of different conformational states, the stability of the polymer formed with these conformational states, and its possible functional role in regulating the actin-myosin interaction.